1. Field of the Invention
The present invention relates to an inertia force sensor that detects an inertia force, particularly an oscillation type inertia force sensor that detects an inertia force using an oscillator.
2. Description of the Related Art
An oscillation type inertia force sensor is employed as, for example, an angular velocity sensor detecting the angular velocity based on inertia force. An oscillation type inertia force sensor includes an oscillator for detecting angular velocity, an oscillation circuit unit supplying a driving signal to the oscillator, and a detection circuit unit detecting the angular velocity of the oscillator. An oscillator includes an electrostatic driving and capacitance detection type, a piezoelectric driving and detection type, or the like. An oscillator includes an oscillating body that oscillates due to the angular velocity, driving means for driving the oscillating body, monitor means for feeding back to an oscillation circuit unit a monitor signal according to the degree of amplitude of the oscillating body (oscillating state of the oscillator), and detection means for providing a detection signal based on the oscillatory displacement caused by a Coriolis force of the oscillating body.
The oscillation circuit unit is formed as a self oscillation circuit of a closed loop with the oscillator as the resonant element to generate a driving signal from a monitor signal according to the degree of the amplitude of the oscillating body and supplies the driving signal to the oscillator to control the drive of the oscillating body. The detection circuit unit generates and outputs an angular velocity detection signal based on the detection signal applied from the detection means of the oscillator. The angular velocity detection signal corresponds to a DC voltage in accordance with the degree of the angular velocity of the oscillator.
The oscillation type inertia force sensor has the detection sensitivity of the angular velocity stabilized by constituting the oscillation circuit unit as a self oscillation circuit. However, there was the case where the detection sensitivity of the angular velocity is rendered unstable due to environmental change such as the ambient temperature and/or power supply voltage of the oscillator or by temporal change. A circuit configuration allowing the detection sensitivity of the angular velocity at an oscillation type inertia force sensor to be stabilized is disclosed in Japanese Patent Laying-Open No. 6-74774 and Japanese Patent Laying-Open No. 2000-283767. The oscillation type inertia force sensor disclosed in Japanese Patent Laying-Open No. 6-74774 includes an oscillator circuit unit having a phase circuit to adjust the phase of a monitor signal fed back from the monitor means of the oscillator, and an automatic gain control circuit (AGC circuit) amplifying the monitor signal. The oscillation type inertia force sensor of Japanese Patent Laying-Open No. 6-74774 has variation in the detection sensitivity of angular velocity exhibited as variation in the voltage of the monitor signal. By virtue of the AGC circuit controlling the voltage of a driving signal supplied to an oscillator based on variation in the voltage of the monitor signal, the detection sensitivity of the angular velocity can be rendered stable.
An oscillation type inertia force sensor disclosed in Japanese Patent Laying-Open No. 2000-283767 has an oscillation circuit unit (feedback amplifier) including an AGC circuit. The AGC circuit generates from a monitor signal a driving signal that renders constant the oscillation of the oscillating body and outputs the driving signal. The oscillation type inertia force sensor of Japanese Patent Laying-Open No. 2000-283767 can oscillate the oscillator always in a resonant state and at a constant oscillation, allowing the detection sensitivity of the angular velocity to be rendered stable even when the ambient temperature changes.
An AGC circuit is formed of a rectifying circuit, a comparison circuit, and a variable gain amplification circuit (VGA circuit). The rectifying circuit rectifies a monitor signal input to the AGC circuit. The comparison circuit compares the voltage of the monitor signal rectified by the rectifying circuit with a reference voltage, and outputs a control signal controlling the degree of the amplification factor (gain) of the VGA circuit based on the compared result. The VGA circuit amplifies the monitor signal applied to the AGC circuit at an amplification factor based on the control signal input from the comparison circuit for output as a driving signal.
According to conventional art, a VGA circuit is employed for controlling the degree of the amplification factor of the VGA circuit by the current/voltage of the VGA circuit, as disclosed in Japanese Patent Laying-Open No. 6-74774. By the physical property of the semiconductor constituting the VGA circuit, the amplification factor varies greatly depending upon the environmental change such as the ambient temperature and/or power supply voltage. The change in the amplification factor of the VGA circuit becomes non-linear to the change of the control signal. FIG. 12 is a circuit diagram representing a configuration of a conventional VGA circuit. The conventional VGA circuit will be described based on the circuit diagram of FIG. 12. The VGA circuit of FIG. 12 includes a MOSFET 100. By applying a control signal (AGCO signal) to the gate terminal of MOSFET 100, the transconductance Gm, Gds of MOSFET 100 is varied to control degree of the amplification factor of the VGA circuit. The amplification factor of the VGA circuit shown in FIG. 12 can be obtained as [{Ra+Rb+(Rc//(Gm+Gds))}/{Rb+(Rc//(Gm+Gds))}] by resistances Ra, Rb, Rc and transconductance Gm, Gds. Since the threshold voltage, the mobility, and the like of MOSFET 100 affecting resistances Ra, Rb, Rc and transconductance Gm, Gds vary greatly depending upon the ambient temperature, the amplification factor of the VGA circuit shown in FIG. 12 greatly varies depending upon environmental change such as the ambient temperature and/or power supply voltage. Furthermore, since the VGA circuit is constituted by the change in the ON resistance of MOSFET 100, the change in the amplification factor becomes non-linear to the change of the control signal. Therefore, when an AGC circuit employing a conventional VGA circuit was employed in the oscillation circuit unit of an oscillation type inertia force sensor, the detection sensitivity of the inertia force (angular velocity) was greatly affected by the individual variation between oscillation type inertia force sensors, the variation in the semiconductor element's resistance and capacitance, MOSFET process parameter fluctuations, and environmental change such as the ambient temperature and/or power supply voltage. Furthermore, when an AGC circuit employing a conventional VGA circuit is used as the oscillation circuit unit of an oscillation type inertia force sensor, a linear relationship could be obtained only for a small range since the change in the amplification factor of the VGA circuit was non-linear to the change in the control signal. There was a problem that the variable range of the amplification factor of a VGA circuit that can be used during module designing becomes smaller.